Top 10 Key Differences Between HashSet and TreeSet in Java

1. Overview of HashSet and TreeSet

Before diving into the differences, let’s briefly review what HashSet and TreeSet are.

1.1 What is HashSet?

A HashSet is a collection that uses a hash table for storage. It implements the Set interface, meaning it does not allow duplicate elements. The elements are unordered and unsorted, making HashSet suitable for scenarios where you need fast lookup, insertion, and deletion.

1.2 What is TreeSet?

A TreeSet is a collection that implements the NavigableSet interface. It uses a Red-Black tree for storage, meaning the elements are stored in a sorted and ordered manner. TreeSet does not allow duplicate elements either, but it is ideal for situations where you need to maintain a natural ordering of elements.

2. Key Differences Between HashSet and TreeSet

2.1 Ordering

• HashSet: Does not maintain any order of elements. The order in which elements are added does not correlate with the order they are stored.
• TreeSet: Automatically orders the elements based on their natural ordering or a specified comparator.

2.2 Performance

• HashSet: Offers constant time complexity O(1) for basic operations like add, remove, and contains, making it much faster when order is not a concern.
• TreeSet: Offers log(n) time complexity for basic operations, as the elements are stored in a tree structure, which takes more time than a hash-based structure.

2.3 Internal Storage Mechanism

HashSet: Uses a hash table internally. Each element’s hash code is used to determine its storage location. If two elements have the same hash code, a technique called chaining or probing is used to handle collisions.

Example Code:

Set<String> hashSet = new HashSet<>();
hashSet.add("Apple");
hashSet.add("Banana");
hashSet.add("Mango");

TreeSet: Uses a Red-Black tree internally. Each element is placed according to its natural order or a provided comparator, ensuring that the tree remains balanced.

Example Code: